Midfielder Joe Allen races a race to be fit for the European Championships, with Stoke boss Michael O'Neill saying he will miss "a number of weeks".
The game in Leuven was Allen's first international appearance since 2019, having previously been out for nine months with an Achilles injury.
"The injury is a bit more severe than we thought initially," O'Neill said.
Allen missed Saturday's friendly win against Mexico and Tuesday's 1-0 World Cup qualifier victory against the Czech Republic, both at Cardiff City Stadium.
Wales are set to begin their European Championship campaign against Switzerland in Baku on Saturday, 12 June.